This is a story about a guy named Mo and how he secretly created his personal paradise in San Francisco Bay.

And it's about a guy named Pete who has done the same thing at Del Valle Reservoir, Lake Chabot and Quarry Lakes.

Back in the day, in the mid-1960s, the San Mateo Bridge was destroyed and replaced with the modern span. A crew with a barge took much of the old concrete roadbed to build the breakwater at Coyote Point Marina in San Mateo.

One day, Mo, a member of the barge crew, talked his buddies into dumping a few loads of the broken-up concrete roadbed along the west edge of the channel of the South Bay, between the San Mateo Bridge and Coyote Point.

In the years that followed, algae and sea grass grew on the big pile of concrete blocks. The underwater structure and its crevices provided habitat for crabs, shiner perch, tomcod, bullhead, grass shrimp, and mussels. In turn, striped bass, halibut, leopard sharks, bat rays, jacksmelt, pile perch ("pogies"), white sea perch ("split tails") and striped sea perch ("surf perch") were all attracted to the site to feed. The rich marine food chain also attracted shorebirds, waterfowl and marine birds.

The few who knew about it called it Mo's Hole. For years, it was the best fishing spot on the bay. We had many 50-fish days, catch-and-release. Until mud covered the concrete blocks, from dumped dredge spoils brought in by incoming tides, Mo's Hole was a local paradise.

Now let me tell you about Pete, as in Pete Alexander. For 10 years, he has persuaded Christmas tree vendors to donate their leftover trees to the East Bay Regional Park District. Volunteers then wire the trees together and anchor them at lakes to create structure, or habitat. An underwater video I posted at sfgate.com/outdoors shows how it works.

Here's the motive of this story: Plans are in the works to demolish Candlestick Park. At least some of that concrete should be planted at different spots in the bay to create marine habitat and fantastic new fishing spots.

If you have a chance to do something that makes a difference, why not do it?